Two people are dead and a toddler is in critical condition after a crash on the 401 near Tilbury.

A 78-year-old woman and a 41-year-old man, both from Toronto, died after their minivan drove into oncoming traffic, striking a westbound tractor trailer and knocking its back wheels off.

A three-year-old boy and a 35-year-old woman are both in critical condition in hospital in London. A fifth occupant of the van was treated for minor injuries.

Ontario Provincial Police say the accident happened west of the Bloomfield Road exit just after 6 p.m. ET Monday.

The 41-year-old man was driving at the time but police can't say yet why his minivan hit the tractor trailer.

Chatham-Kent OPP continue to investigate. Const. Troy Carlson said the OPP hope to eventually speak with the woman in hospital.

"Obviously the three-year-old won't be able to provide anything. The 35-year-old is still in critical condition. So it'll be a matter of speaking to them to determine what possibly the cause was," Carlson said.

OPP say the 50-year-old driver of the trailer was able to bring his vehicle to a stop. The force of the impact was so strong, the rear four wheels of the tractor trailer were sheered off. The sparks from the dragging trailer caused a small grassfire.

The driver of the tractor-trailer was uninjured.

The Highway 401 was closed in both directions for hours while police investigated.